1. Field of the Invention
The present invention relates to a contact image sensor. More specifically, the invention relates to a contact image sensor, image scanner, facsimile machine, copying machine, or the like capable of correcting dark output for each pixel with ease and high precision and capable of switching between different resolution levels.
2. Description of the Related Art
There have previously been proposed contact image sensors in which dummy pixels are created from as many photodiodes, which are of the same type as used for light receiving devices. The photodiodes within a block are provided for dark correction separate from a light receiving device array and shielded against light to avoid incidence of light from above the pixels.
FIG. 20 is a circuit diagram showing an integrated circuit for a prior art contact image sensor. In this prior art, dummy pixels 121″ for dark correction are created from n (n represents the number of pixels within one block) photodiodes, which are each denoted by P0.j (j=1 through n) and are of the same type as used for light receiving devices, which constituted a dummy pixel array 121′ separate from a light receiving device array 111. Provided for dark correction, the dummy pixels 121″ are shielded against light to avoid incidence of light from above the pixels. The photodiode P0.j of each of the dummy pixels 121″, is connected to the drain electrode side of its switching device, namely, a thin film transistor T0.j (j=1 through n) of a dummy pixel charge transfer unit 122. A gate electrode of the thin film transistor T0.j is connected through a common gate line G0 to a gate pulse generating circuit 116, from which a gate pulse φ G0 is outputted to control on/off of the thin film transistor T0.j. As shown in FIG. 20, a source electrode of each thin film transistor T0.j, connected to the respective dummy pixels 121″, is connected through a multilayer wire 113 to a common signal line 114 similar to a source electrode of a thin film transistor T0.j . Wiring capacitance CLJ (j=1 through n) is formed in each common signal line 114, so that electric charges outputted from light receiving devices 111″ or from the dummy pixels 121″ are transferred to and accumulated in the respective wiring capacitance. A driver IC 115 reads the electric potential of the common signal line 114 after the electric potential is raised by the electric charges transferred to and accumulated in the wiring capacitance. The driver IC 115 then outputs, in time series, image information via an output terminal 117. Since the dummy pixels 121″ are shielded against light, electric charges generated in each of the dummy pixels 121″, in the dark state when the thin film transistor T0.j is turned on, are transferred to and accumulated in the wiring capacitance CLj of the common signal line 114 to be read as the voltage value by the driver IC 115. The voltage value read is outputted as image information in the dark state in each common signal line 114 that includes the wiring capacitance CLJ (see JP 06-113136 A, for example).
In a conventional image sensor IC structured as above, there is a difference in load capacitance between light receiving devices because dummy pixels are shielded against light. Furthermore, since the dummy pixels in the conventional image sensor IC are arranged separate from the light receiving devices, which means that different common signal lines and different pixels are used, it is difficult to make the dark level match while factoring in unbalance due to a difference in wiring capacitance level between common signal lines, fixed pattern noise inherent to individual ICs due to a voltage drop that results from power supply line resistance unique to long ICs, switching noise, and temperature characteristics.